The invention relates to a process for producing surface-decarburized steel sheets or plates by heating a pile or package of alternate layers of a decarburizing agent and the steel sheet(s) or plate(s).
Processes for the surface-decarburization of steel sheets or plates are known. According to such a process carbon and other impurities can be removed by heating the sheets or plates in an atmosphere containing free hydrogen in the presence of aluminum silicide, calcium silicide or the like. According to another process the sheets or plates are decarburized by annealing them in a hydrogen atomosphere, wherein the hydrogen consumed in the decarburization process by the formation of methane is regenerated with the help of metallic calcium that is strewn about the sheet surface.
Surface-decarburization processes are also known, from German Auslegeschrift No. 1,193,527 and Austrian Pat. No. 223,224, in which the sheets or plates to be decarburized are alternately put together with calcium foils or with layers of comminuted calcium aluminum alloy to form a pile. The pile is then heated to a temperature of between 700.degree. and 1200.degree. C in the absence of hydrogen. In the decarburizing-agent layers, alkali earth oxides, such as MgO, CaO, SrO or BaO, can be included in order to counteract the possibility of sintering. Because of the great affinity of the metallic calcium or the calcium-aluminum alloy, respectively, for oxygen, this process for decarburizing or scaling must be carried out in an inert protective atmosphere of noble gases, such as helium or argon. This is complicated and expensive. A further disadvantage of using alkali earth metals or their alloys as decarburizing agents is that during heating firmly adhering coatings form, which lead to an uneven decarburization and which subsequently are very difficult to remove. When the layer of the decarburizing agent is not quite uniform, differing decarburization depths are obtained in the annealing treatment or the decarburization can even be absent in certain places.